This invention relates to an improved electron beam convergence device and an improved purity adjustment device for color television tubes.
One type of cathode ray tube conventionally used in a color television receiver includes an electron beam gun generally designated as an "in-line gun" for developing three electron beams disposed on the screen in linear array. The cathode ray tube also includes a slotted shadow mask and a screen formed of red, green and blue phosphor dots. The shadow mask is operable to aid each electron beam in impinging only upon the phosphor dots of an assigned hue. While the center beam is used as a reference, the outer beams are moved in a manner so that they will cross the reference (center) beam at the shadow mask. This operation is generally designated as "converging" which requires selected movement of one of the beams, but not the others. If when a selected beam is moved, another beam moves along with it, such "chasing" results in difficulty in achieving proper convergence.
It is, therefore, an object of the present invention to alleviate "chasing" and to permit proper convergence by providing a device which permits movement of only one beam at a time.
One prior art type of convergence device for in-line guns comprises three pairs of rings carried by a member which attaches to the neck of the cathode ray tube. The first pair of rings has three pole pairs, with each of the two rings being rotatable in opposition to each other and also movable as a unit. The second pair of rings is similar to the first pair of rings, but each ring has two pole pairs instead of three pole pairs. The third pair of rings are for purity adjustment. Each ring only has one pole pair and each is rotatable in opposition to the other.
It has been found that the aforementioned convergence device is relatively difficult to operate to converge properly.
Another type of convergence device for in-line guns is sold by Schwalm Electronics, Inc., Highland Park, Ill., and comprises four parallel ferrite rods, each of which is rotatable. Two of the rods are required to control vertical beam positioning and the other two rods are required to control horizontal beam positioning.
It is an object of the present invention to provide a convergence device which is simple in construction and operation, and is economical to manufacture.
A further object of the invention is to provide a convergence device in which rod-like magnets may be both axially and rotatably moved to adjust one of the electron beams at a time.
In order for each electron beam to impinge only upon its own corresponding phoshor dots, "purity" adjustments are often necessary. It has been found that most effective purity adjustment is achievable when the magnetic lines of force provided by a purity magnet device are relatively straight with respect to the cathode ray tube. To this end, it is an object of the invention to provide a purity adjustment device which is simple in construction, efficient to manufacture and provides substantially straight magnetic lines of force with respect to the cathode ray tube.
Other objects and advantages of the present invention will become apparent as the description proceeds.